Allow for changing surface ID in WaitForSurfaceReady

This CL is experimental, and aims to see if a changing surface id for
the root view could be at the heart of the flakey tests reference by
the associated bug.

One flakey test is re-enabled to provide some initial data.

The surface id idea is cherry-picked from kinuko@'s CL at
https://chromium-review.googlesource.com/c/chromium/src/+/1126598 .

Bug: 833380
Change-Id: I3cda4f5214a5bf19e711f95303a6b65b4ff2410c
Reviewed-on: https://chromium-review.googlesource.com/1172723
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583027}

content/browser/site_per_process_hit_test_browsertest.cc

@@ -1340,16 +1340,8 @@ IN_PROC_BROWSER_TEST_P(SitePerProcessEmulatedTouchBrowserTest,
   RunTest(ScrollBubbling);
 }
 
-#if defined(OS_LINUX)
-// Flaky: https://crbug.com/833380
-#define MAYBE_EmulatedTouchPinchGoesToMainFrame \
-  DISABLED_EmulatedTouchPinchGoesToMainFrame
-#else
-#define MAYBE_EmulatedTouchPinchGoesToMainFrame \
-  EmulatedTouchPinchGoesToMainFrame
-#endif
 IN_PROC_BROWSER_TEST_P(SitePerProcessEmulatedTouchBrowserTest,
-                       MAYBE_EmulatedTouchPinchGoesToMainFrame) {
+                       EmulatedTouchPinchGoesToMainFrame) {
   RunTest(PinchGoesToMainFrame);
 }
 

content/public/test/hit_test_region_observer.cc

@@ -50,8 +50,7 @@ SurfaceHitTestReadyNotifier::SurfaceHitTestReadyNotifier(
 
 void SurfaceHitTestReadyNotifier::WaitForSurfaceReady(
     RenderWidgetHostViewBase* root_view) {
-  viz::SurfaceId root_surface_id = root_view->GetCurrentSurfaceId();
-  while (!ContainsSurfaceId(root_surface_id)) {
+  while (!ContainsSurfaceId(root_view->GetCurrentSurfaceId())) {
     // TODO(kenrb): Need a better way to do this. Needs investigation on
     // whether we can add a callback through RenderWidgetHostViewBaseObserver
     // from OnSwapCompositorFrame and avoid this busy waiting. A callback on